1. Field of the Invention
The present invention relates to a semiconductor photodiode mainly used for optical communications. (Hereafter a semiconductor photodiode is referred to as a xe2x80x9cPDxe2x80x9d.) Particularly the present invention relates to a surface-mounting type PD in which signal light enters from a bottom surface of the PD, and an optical receiver using such PD.
2. Definitions
In this specification and claims, the terms of xe2x80x9ctop surfacexe2x80x9d, xe2x80x9cbottom surfacexe2x80x9d and xe2x80x9cside surfacexe2x80x9d of a PD are defined as follows:
The xe2x80x9ctop surfacexe2x80x9d of PD means the top surface of a laminated layer farthest from a substrate.
The xe2x80x9cbottom surfacexe2x80x9d of a PD usually means the bottom surface of substrate. Some semiconductor PDs have a few layers laminated on the bottom surface of its substrate. For example, in the case that a metallized layer for contact is formed on the bottom surface of a substrate, the bottom surface of a PD means the metallized surface, and not the bottom surface of the substrate itself.
The surface-mounting type PDs are classified into three types in accordance with the direction of light incident thereon: a top-incidence type, in which light enters from the top surface, a bottom-incidence type, in which light enters from the bottom surface, and a sidle-incidence type, in which light enters from the side surface.
The device in which the light enters from the top surface is referred to as an xe2x80x9cTop-illuminated PDxe2x80x9d. The device in which the light enters from the bottom surface is referred to as a xe2x80x9cBottom-illuminated PDxe2x80x9d. The device in which the light enters from the side surface is referred to as a xe2x80x9cSide-illuminated PDxe2x80x9d.
3. Description of the Background Art
In order to meet the development of optical communications high-sensitivity and easy handling PDs have been required. As the first conventional example, FIG. 1 shows a surface-mounting type Bottom-illuminated PD that was proposed in German Patent No. DE 35 43 558 C2 (Ref. No. 1). As a p-electrode is formed directly on the top surface of a Bottom-illuminated PD, it enables the diameter of the light receiving area to be small, and the shape of the p-electrode need not be a ring-type. While maintaining an area sufficient to receive light, the capacitance of a pn-junction can be made small, and thereby a high-sensitivity and high-speed-responsivity device can be obtained. In addition, such Bottom-illuminated PD is the most suitable to apply the surface-mount technologies because the light enters from a bottom surface. In other words, it is possible to structure such that is fixed facing upward at the end of a V-groove so as to receive the light from the bottom surface.
A Bottom-illuminated PD 1 includes a wide n-type portion 2 and a narrow p-type domain 3. An interface between the n-type portion 2 and the p-type domain 3 is a pn-junction 4. The n-type portion 2 includes an n-type substrate and an n-type epitaxial layer. A p-electrode having no aperture is formed on the p-type domain 3. A ring-shaped n-electrode is formed on the bottom surface of the substrate. An Si-substrate 5 is a rectangular plate to be used for making a SM device. A V-groove 6 is formed along a center axial line of the substrate 5. The V-groove 6 can be formed by the anisotropy-etching method. An optical fiber 7 is laid on the V-groove 6, and then the fiber is fixed thereon. The ring-shaped n-electrode of the Bottom-illuminated PD is formed on the Si-substrate 5. A p-electrode in a top surface 9 is connected to preamplifiers by wire bonding, that is not illustrated in FIG. 1.
An incident light 8 that is emitted from the optical fiber 7 and travels along the V-groove 6, is reflected at a mirror 11, and after passing through the ring-shaped n-electrode, the light 8 enters from a bottom surface 10 in the n-type portion 2 and progresses to the pn-junction 4, where the light 8 generates photocurrent at the pn-junction.
The second conventional example is shown in FIG. 2, that is a cross-sectional view of a p-i-n-PD, having an InGaAs photo-detecting layer, which has been frequently used in recent optical communications. (Ref. No. 2: U.S. Pat. No. 5,365,101) An n-InP buffer layer 13, an n-InGaAs photo-detecting layer 14 and an n-InP window layer 15 are epitaxially grown in this order on an n-InP substrate 12. A window layer used in this specification and claims is also called a cap layer in this field. P-type dopants are diffused from the top surface of the window layer 15 to the central and peripheral areas thereof to form a p-type domain 3 and a shield domain 16. Interfaces between p-type domains and an n-type domain are pn-junctions 4. A passivation film 17 is formed on a top surface in order to protect the pn-junctions 4. A p-electrode 19 having no aperture is formed on the center of the p-type domain 3. A ring-shaped n-electrode 18 is formed on a bottom surface of the n-InP substrate 12. As this is the Bottom-illuminated PD, it has an aperture part for receiving the incident light 8 in the center of the bottom surface of the InP substrate 12.
The conventional Bottom-illuminated PDs shown in FIGS. 1 and 2 have an n-type InP substrate that is not always good for transmittance from the standpoint of light transmission through the substrate.
Higher transmittance of the n-InP substrate is necessary to improve the sensitivity of the Bottom-illuminated PD.
An InP substrate containing 3xc3x971018 cmxe2x88x923 to 10xc3x971018 cmxe2x88x923 of tin (Sn) or sulfur (S) has been generally used as an n-type substrate. However, such substrate absorbs from 10% to 20% light. Large amounts of these n-type dopants must be doped into the substrate to raise the resistivity of the n-type substrate. The increase in the absorption by n-type dopants results in the increased absorption in the substrate. If the sulfur density is lowered to 1xc3x971018 cmxe2x88x923, the transmittance of the substrate can be considerably improved, but there are drawbacks such as an increase in the resistivity of the substrate, or an increase in the crystal-defect.
FIG. 3 is a diagram showing the relationship between transmittance and wavelength in the case of an S-doped n-InP and an iron (Fe)-doped semi-insulated (SI)-InP substrates, each having a thicknesses of 350 xcexcm. The abscissa axis is the wavelength (nm), and the ordinate axis is the transmittance.
Sulfur is an n-type dopant in this case. The more dopants are contained, the more light is absorbed. This is because light absorption is caused mainly by the dopants. The longer the wavelength, the less the absorption becomes. However, when the sulfur density decreases, the minimum absorption wavelength moves to about 1.3 xcexcm.
For example, in the case of carrier density of 6.5xc3x971018 cmxe2x88x923, transmittance is about 0.75 at a 1.3 xcexcm optical wavelength. In the case of carrier density of 3.3 xc3x971018 cmxe2x88x923, transmittance is about 0.87 at the same wavelength. In the case of carrier density of 1.0xc3x97108 cmxe2x88x923, transmittance becomes 0.96. In other words, optical absorption still remains 0.04 in this case. On the other hand, in the case of carrier density of 1.0xc3x971018 cmxe2x88x923 Fe-doping, transmittance is 0.98 and absorption can be reduced to 0.02 at the same wavelength.
Conventionally low resistance n-type or p-type substrates have been used for PDs of the Si-series, GaAs-series or InP-series. As an electrode has been formed on a bottom surface of the substrate, the photocurrent has had to pass through the substrate. If the substrate has high resistance, photocurrent can not flow easily and response speed becomes slow. Therefore, the substrates have been made of low-resistivity p-type or n-type crystals.
On the other hand, Fe makes a deep energy level in a forbidden band in the InP crystal. As the deep energy level captures electrons, the movable electron density is decreased. So, an Fe-doped InP crystal becomes highly resistant. In other words, the Fe-doped substrate has insulating or semi-insulating property. An electrode therefore cannot be formed on a bottom surface of the Fe-doped substrate. This is the reason why an Fe-doped substrate has been rarely used for a PD substrate.
M. Makiuchi, H. Hamaguchi, O. Wada, and T. Mikawa, published xe2x80x9cMonolithic GaInAs Quad-p-i-n Photodiodes for Polarization-Diversity Optical Receiversxe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 6, JUNE 1991, p535-536 (Ref. No. 3) for an example of a Bottom-illuminated PD. However this type of device has not been used practically so far because of its problems in terms of production and quality.
As the third conventional example, a Side-illuminated PD is shown here, although it was proposed for an object which is different from that of the present invention.
FIG. 4 shows an oblique view of the Side-illuminated PD (Ref. No. 4: Hideki Fukano, Atsuo Kozen, Kazutoshi Kato, and Osaake Nakajima, xe2x80x9cHigh-Responsivity and Low-Operation-Voltage Edge-Illuminated Refracting-Facet Photodiodes with Large Alignment Tolerance for Single-Mode Fiberxe2x80x9d, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 5, MAY 1997, p894-896).
This PD""s layer-structure from the lowest layer is:
an SI-InP substrate 170,
an n+-InP buffer layer 171,
an n+-InP contact layer 172,
an nxe2x88x92-InGaAs layer 173,
an n-InP layer 174,
a p+-InP layer 175,
a p-electrode 176, and
an n-electrode 177 (The n-electrode 177 is placed on the InP contact layer 172).
This PD shows a dissymmetric tip-shaped PD. The n-electrode 177 is formed on the InP contact layer 172 that is exposed, with a portion of the n-InP layer 174 and the n-InGaAs layer 173 being removed by etching. A rectangular part of the n-InP layer 174 becomes the p-InP layer 175 by means of zinc (Zn) diffusion. The p-electrode 176 is formed on the p-InP layer 175. A side surface of the substrate becomes a downward slant surface 179. Alight 178 enters into the slant surface 179 horizontally. The light, after being refracted upward at the side surface, reaches a pn-junction as a refraction light 180. This is a Side-illuminated PD. The light enters into the neighboring pn-junction of the photo-detecting layer by being refracted upward at the side surface of a layer beneath the photo-detecting layer. This PD is different from the present invention in that light enters the side surface of PD. And this PD is not a Bottom-illuminated PD, but rather a top-surface-electrode type PD. As the structure is complicated, there are many problems such as cost to be solved. This type of PD is defined as a xe2x80x9ctop-electrodexe2x80x9d PD in order to distinguish it from a conventional top or bottom surface-mounting type PD that has an electrode on the top or bottom surface of the PD, respectively.
The present invention relates to a high-performance Bottom-illuminated PD having following structures, which is the most suitable for applying surface-mount technologies, and also relates to an optical receiver using such PD. A PD according to one embodiment of the present invention comprises an insulating or an SI-substrate, such as an Fe-doped InP single crystal, a highly doped n-type buffer layer, a less-doped photo-detecting layer and a window layer, one laminated on another in the enumerated order, and a conductive domain formed in the central part of the top layer, a p-electrode formed on the conductive domain, and an n-electrode formed on the buffer layer being exposed by etching.
That is, the present invention is characterized in that the n-electrode is laid on the partly exposed buffer layer. The buffer layer is a thin film layer grown on the SI-substrate, being interposed between the substrate and the photo-detecting layer in order to improve the crystal matching with the photo-detecting layer. As the photocurrent flows in this layer, the layer is low resistive, namely, highly doped. In the case where an Fe-doped n-InP single crystal is used for the substrate, the buffer layer is a highly-doped InP film. A part of the upper layers, such as a photo-detecting layer and a window layer, are removed for exposing a part of the buffer layer in order to form an n-electrode onto the exposed buffer layer.
A Bottom-illuminated PD according to one embodiment of the present invention relates is characterized by the following structures:
1. A substrate is made of crystal having high transmittance property, such as SI Fe-doped InP single-crystal.
2. A part of the upper layers including the photo-detecting layer, etc. are removed so that a part of the buffer layer is exposed.
3. An n-electrode is formed on the exposed buffer layer.
In this summary and the following description in this specification, the SI substrate is mainly described as a substrate of the present invention, however, the insulating substrate is also applicable as a substrate of the present invention.